The sliding filament theory

1Shorter, but nothing shrinks

In 1954 two pairs of scientists working separately, Andrew Huxley with Rolf Niedergerke, and Hugh Huxley with Jean Hanson, published the same surprising observation. When a muscle fibre contracts, its dark A bands stay exactly the same length. Only the light I bands and the H zones get narrower. If the filaments themselves were shrinking, every band would shrink. Instead, the thick and thin filaments must be sliding past each other, increasing their overlap. This is the sliding filament theory, and this lesson follows it from a nerve impulse down to a single myosin head pulling on actin.

2The evidence: which bands change?

3From nerve impulse to calcium release

1. An action potential arrives at the end of a motor neurone at the neuromuscular junction. Ca²⁺ ions enter the neurone's terminal, and vesicles release acetylcholine (ACh) into the synaptic cleft by exocytosis.
2. ACh diffuses across the cleft and binds to receptors on the motor end plate. Sodium ion channels open, the sarcolemma depolarises, and an action potential spreads across the fibre's surface. Acetylcholinesterase in the cleft quickly breaks down ACh, so each impulse causes only one response.
3. The action potential travels down the T-tubules into the fibre. Where each T-tubule touches the sarcoplasmic reticulum, voltage-sensing proteins open calcium channels in the SR membrane.
4. Ca²⁺ ions flood out of the SR into the sarcoplasm around the myofibrils.

4The cross-bridge cycle

At rest, tropomyosin lies over the myosin-binding sites on actin. When Ca²⁺ binds to troponin, troponin changes shape and pulls tropomyosin away from the binding sites. The myosin heads can now attach, and each head goes round a cycle:

Each power stroke moves the thin filament only about 10 nm, but each thick filament has hundreds of heads cycling out of step with one another, so some are always attached and the filament never slips back. Because the heads on each half of a thick filament face opposite ways, both sets of thin filaments are pulled towards the M line and the Z lines are drawn together.

5Relaxation

When the motor neurone stops firing, no more ACh is released and the sarcolemma repolarises. Ca²⁺ ions are actively pumped back into the SR, which uses ATP. Troponin loses its Ca²⁺, tropomyosin slides back over the binding sites, and cross-bridges can no longer form. The muscle does not lengthen by itself: it is pulled back to its resting length by the contraction of its antagonist or by gravity.

6Grading the force

A single impulse makes a fibre contract fully: a fibre's response is all-or-nothing. The whole muscle produces a graded force because the nervous system controls how many motor units are active at once and how often they fire. Muscles used for precise movements, such as those that move the eye, have small motor units of a few fibres each; large postural muscles have motor units of hundreds or thousands of fibres.

7Supplying the ATP

ATP is used three times in each contraction: to detach and re-cock each myosin head, and to pump Ca²⁺ back into the SR. A muscle holds only enough ATP for a few seconds of hard work, so it is regenerated in three ways:

• Creatine phosphate: the enzyme creatine kinase transfers a phosphate group from creatine phosphate to ADP: $\text{creatine phosphate}+\text{ADP}\to \text{creatine}+\text{ATP}\backslash text\{creatine\; phosphate\}\; +\; \backslash text\{ADP\}\; \backslash rightarrow\; \backslash text\{creatine\}\; +\; \backslash text\{ATP\}$. This is the fastest source and powers the first few seconds of a sprint; the store is rebuilt later from ATP when the muscle is resting.
• Anaerobic respiration: glycolysis of glucose from glycogen gives 2 ATP per glucose and produces lactate (Unit 4). It is fast but inefficient.
• Aerobic respiration in the mitochondria gives far more ATP per glucose and can also use fatty acids, but it depends on the supply of oxygen, so it is slower to increase.